1. Field of the Invention
The present invention relates generally to semiconductor laser devices, and particularly to a semiconductor laser device suitable for use in optical information processing.
2. Description of the Prior Art
Semiconductor laser devices have many features such as small size and high efficiency, capability of direct modulation by driving current, and the like, and they have been widely utilized as light sources for optical communication or optical information processing in recent years. In order to be utilized for these purposes, little fluctuation of light output, i.e., little light intensity noise is necessary. What is especially important is smallness of the light intensity noise in case the outputted light is reflected and returned at the coupling with the optical system.
A conventional semiconductor laser, for instance, a semiconductor laser as shown in FIG. 1, comprises a substrate 1 of GaAs crystal on which a channel 11 is formed, and includes
n--Ga.sub.1-y Al.sub.y As (y.perspectiveto.0.4) as a clad layer 2, PA1 n--Ga.sub.1-x Al.sub.x As (x.perspectiveto.0-0.2) as an active layer 3, and p--Ga.sub.1-y Al.sub.y As (y.perspectiveto.0.4) as a clad layer 4.
As facets of a resonator, cleaved faces of the crystal chip are utilized, and a cleaved face is used without a cover or is covered with dielectric layers of SiO.sub.2 or Al.sub.2 O.sub.3 with a thickness of .lambda./2n, where n is the refractive index and .lambda. is the wavelength of oscillation of the laser. Incidentally, the conventional laser shown in FIG. 1 further comprises an overriding layer of n--GaAs, a Zn diffused region 51, a positive side electrode 6 of Ti/Pt/Au and a negative side electrode 7 of Au--Ge--Ni.
The current vs. light output characteristic and the current vs. noise characteristic of the conventional semiconductor laser is as shown in FIG. 2. Also, the temperature dependency of the S/N ratio for the case where 0.1-5% of the light output returns by reflection is shown in FIG. 3. As is shown in these graphs, noise frequencies of 2-12 MHz are measured with a bandwidth of 300 KHz. Such noise characteristics are harmful for actual use, especially for use as a light source for an optical type video disk player, thereby disabling actual usage.